Error message sending emails

Dear all,
at the moment, we don't handle our mail server by ourselves; done by 3rd comany.
For some days we have the problem that we cannot send any emails to our affiliate in US. we always get the pasted error message (end of this post)
Anyway our 3rd party provider tells us it's the fault of US side.
Actually I don't think so.

Am I right that "450 4.7.1 Client host rejected: cannot find your hostname," indicates that the "reverse host lookup" failed and that this could be solved by some settings by our 3rd party mailhoster?

Can anyone help me and explain some details what I can tell them about the problem?

Thanks a lot in advanced

Edit1 ) Unfortunately
I got the message " We are sorry, new user accounts are not permitted to create posts containing E-Mails. Please review our forum rules for more information." thats why I attached a screen shot instead of the test

Edit2) This time:
"We are sorry, new user accounts are not permitted to create posts containing URLs. Please review our forum rules for more information." - Not easy to get a post I try this one

Some servers are configured to match up the forward and reverse lookups for servers before accepting mail. I am not 100% sure on whether or not the lookup is done based on the hostname provided in the HELO command, or the actual hostname of the server itself. This could very well be software/configuration dependent.

For example, if we were sending mail from mail.example.com the transaction would begin:

HELO mail.example.com

The receiving server looks up the A record for mail.example.com and gets 1.2.3.4

The receiving server now looks up the reverse record [PTR] for 1.2.3.4, [4.3.2.1.in-addr.arpa] and expects to receive mail.example.com

If the records match up the connection is accepted, and the transaction continues.

If the records do not match, or a reverse record is not set, you recieve the bounce messages that you have been getting.

The best thing to do in this case would be to contact the administrators of the server and have them look into it. Keep in mind the following:

Only the owner of the IP address can set the reverse record.

Only one reverse record can be set for a given IP address, and chances are it's going to be a shared IP.

Hi and thanks for your reply,
the problem is that the administrator seems to be a fool and just says it's not their fault I need to contact US side.

Even US side confirmed that this is nonsense. They could whitelist our domain, but would receive more spam. Of course nobody want's that. So I need to find some idea or hint why it's 100% the fault of the mail admin and his wrong settings.

You will have to check for PTR record. In some cases, some email services reject emails if PTR record is not properly set in mail server. Also, make sure that the email server IP is not blacklisted. You can check it from the below mentioned URL:

Hard to say, the 4xx response is fro the US server and it does not like something about your domain name. You can test all the DNS configurations to see if the DNS for your domain is configured properly. go to mxtoolbox.com and use those tools. They are free, and may be able to shed some light on this. PM me if you need some help.